Applicants claim priority to German patent application 200 16 803.7 filed Sep. 29, 2000.
1. Field of the Invention
The invention concerns a filter device having filter elements made of ceramic material which are combined into at least one filter element group in which the filter elements lie side by side next to one another and can be flowed through in parallel.
2. Description of the Related Art
For hot gas filtration, for example the filtration of diesel engine exhaust gases, increasing use is being made of filter elements made of porous ceramic material; particularly successful here is the SiC ceramic material that is described in EP 796 830 A1. It is chemically stable and has high thermal conductivity, temperature resistance, and thermal shock resistance. As is evident from WO 93/13303 and EP 1 013 328 A2, filter elements of this kind are combined into one filter element group or several of them, in which the filter elements lie side by side next to one another and are flowed through in parallel. Located between the filter elements are contact layers made of material expanded by the action of heat. The filter element groups are surrounded by housings that can also be replaceable.
For use on diesel engines, it is important for the soot particles deposited on or in the filter element to be removed at regular intervals so that the flow-through resistance, in particular in the filtrationally effective walls of the filter element, does not become too high. One known cleaning method consists in heating the filter elements, by application of an electric current, to such an extent that the soot particles ignite and burn off (cf. WO 93/13303, EP 0 943 593 A1). For this purpose, the ceramic material can be given a suitable electrical resistance (cf. EP 0 796 830 A1). Particular electrical terminalsxe2x80x94as described, for example, in EP 0 943 593 A1xe2x80x94are necessary to ensure current transfer with as little resistance as possible.
U.S. Pat. No. 4,505,107 describes a filter device that contains, in a filter housing, a filter element made of porous ceramic foam. Provided on its end face is a heating device that comprises a heating grid made of electrically conductive ceramic material that is equipped with a plurality of electrical terminals, each two terminals being connected to an electrical circuit. In this fashion, the heating grid can be acted upon by current, and thus heated, sequentially in sections. Instead of the heating grid it is also possible to provide a plurality of heating elements with a ceramic coating, distributed over the end surface of the filter element, which are also acted upon sequentially by electric current.
Electrical cleaning or regeneration results in a not inconsiderable installation and control outlay. Efforts are therefore being made to dispense with this type of cleaning and to perform cleaning by way of the exhaust gas itself, for example by elevating the exhaust gas temperature or by artificially generating NO2 that transforms the soot, by higher oxidation, substantially into CO2. This type of cleaning cannot, however, be reliably achieved in all operating states. In particular at low engine speeds, under low load, or in short-distance driving, the exhaust gas cannot be conditioned so as to bring about combustion of the soot. If these operating conditions persist for a long period, they can cause clogging or at least a considerable exhaust back pressure that results in decreased performance and increased fuel consumption.
It is therefore the object of the invention to configure a filter device of the kind cited initially in such a way that soot cleaning is possible even under operating conditions in which cleaning by way of the exhaust gas is not successful.
This object is achieved, according to the present invention, in that only some of the filter elements of each filter element group have electrical terminals for connection with an electrical energy source, and at least that/those partial element(s) is/are made of electrically conductive ceramic material.
The basic idea of the invention is thus to provide electrical cleaning but to limit it to some of the filter elements and accordingly to keep the outlay for equipment comparatively low. This is based on the consideration, already belonging to the invention, that the exhaust gas volume is low in precisely those operating states in which combustion of the carbon by way of the exhaust gas is not successful, and accordingly even a partial cleaning is sufficient to ensure a sufficiently low exhaust gas pressure. Complete regeneration occurs when the diesel engine has warmed up and greater load states are demanded. The two cleaning methods thus complement one another with little additional outlay for equipment. For the other filter elements, furthermore, their electrical conductivity does not need to be taken into account.
In an embodiment of the invention, provision is made for the first filter element(s) to be arranged at the center, since energy conversion there is particularly effective. The possibility also exists, however, for the first filter elements to be arranged in distributed fashion in the filter element group, greater flexibility being thereby achievable. In this case the possibility exists of cleaning the filter elements alternatingly by way of a corresponding circuit.
The invention does not exclude the fact that the second filter elements are also made of electrically conductive ceramic material. This is a good choice in particular if the second filter elements resting against the first filter element(s) are in electrically conductive contact with the first filter element(s). Concurrent heating of the second filter elements can thereby be brought about. By appropriately designing the electrical conductivity of the first and second filter elements and the transition between the two, the concurrent heating can be adapted in customized fashion to respective conditions. The possibility also exists, however, of producing the second filter elements from an electrically nonconductive material, if active concurrent heating is not desired. In that case concurrent heating is brought about by thermal conduction.
According to a further feature of the invention, provision is made for the thermal conductivity of the second filter elements to be greater than or less than that of the first filter element or elements. The concurrent heating of the second filter elements can thereby be adjusted accordingly.
The filter elements can be arranged in direct contact with one another. For an assemblage, it is advisable to arrange contact layers between the filter elements. The electrical and thermal resistance can be influenced to the particular degree desired by way of the material and thickness of the contact layers. Contact layers that are made of the same ceramic base material as the filter elements are particularly suitable for this purpose.
If concurrent heating is desired, the contact layers can be designed to be electrically conductive. Contact layers like those described in EP 0 943 593 A1 have proven successful. The electrical conductivity of the contact layers should be lower than that of the first filter elements.
The contact layers can be configured in such a way that they directly connect the filter elements. Separate contact layers with no material connection to the filter elements can, however, also be provided. In that case the filter elements are held together by a housing that surrounds them.
The thermal conductivity of the contact layers can be of the same order of magnitude as that of the first or second filter elements.
According to a further feature of the invention, provision is made for the filter elements to comprise, next to and alternating with one another, inflow conduits and outflow conduits that are separated by porous, filtrationally effective longitudinal walls, the inflow conduits being open on the inflow side and closed on the outflow side, and the outflow conduits being closed on the inflow side and open on the outflow side. Inflow and outflow conduits having a rectangular, in particular square, cross section have proven successful. In this context, the outer walls of the filter elements should be of particle-tight, in particular gas-tight, configuration. The filter elements can have a rectangular, square, oval, round, and/or shell-shaped cross section.
The filter element group(s) is/are advantageously surrounded by a housing having a gas inlet and gas outlet; each filter element group can be surrounded by a separate housing if multiple filter element groups are present A corresponding embodiment is evident, for example, from EP 1 013 328 A2.
The invention is illustrated in more detail, with reference to exemplary embodiments, in the drawings, in which: